spec: update assignments api for post policies
spec: update assignments api for post policies

Change-Id: I08e740f4e3edb2c2e6ffc412a13868fe96f6b9c5
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/229534
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Robin Kuss <0b206b54367b2de0334896ea45826f0e3194bd35@instructure.com>
QA-Review: Jeremy Neander <20252aad5efdc666f083d82ab63ed0f9f72a043e@instructure.com>
Product-Review: Jeremy Neander <20252aad5efdc666f083d82ab63ed0f9f72a043e@instructure.com>

instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,djbender/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,djbender/canvas-lms